You might be measuring the frame of the escape
hatch. most of them the opening is 14X22 without the frame.
You do have choices...
1] You could keep the escape
hatch and place the fan elsewhere.
2]You could remove the escape hatch and install the fan in the center of the opening, using a piece of Fiberglas sheet on each side, then place a light
on each side or fill in with ratfur.
3] You could modify a fantastic fan and install it still using the escape hatch lid and frame.
I like the second option, I live in Illinois and I would be glad to give you some excellent fiberglass panel(s) to accomplish the job.
PM me to discuss details as to what I am thinking would make a professional looking mod.